FIG. 9 is a diagram illustrating the constitution of a conventional power source for driving a magnetron. In FIG. 9, an alternating current from a commercial power source 11 is rectified into a direct current through a rectifier circuit 13, is smoothed through a choke coil 14 and a smoothing capacitor 15 on the output side of the rectifier circuit 13, and is fed to the input side of the inverter 16. The direct current is converted into a desired high frequency (e.g., 20 to 40 kHz) by turning the IGBT in the inverter 16 on and off. The inverter 16 is driven by the IGBT that switches the direct current at a high speed and by an inverter control circuit 161 that drives and controls the IGBT, whereby a current flowing through the primary side of a booster transformer 18 is switched to be on/off at a high speed.
Input signals to the control circuit 161 are detected by detecting a primary side current of the rectifier circuit 13 by using a CT 17, and the detected signals are input to the inverter control circuit 161 and are used for controlling the power of the inverter 16. Further, a temperature sensor (thermistor) 9′ is attached to the cooling fins for cooling the IGBT, and the temperature data detected by the temperature sensor are input to the inverter control circuit 161 to control the inverter 16.
In the booster transformer 18, a high-frequency voltage output from the inverter 16 is applied to the primary winding 181, and a high voltage proportional to a turn ratio is obtained on the secondary winding 182. Further, a winding 183 of a small number of turns is provided on the secondary side of the booster transformer 18 and is used for heating a filament 121 of a magnetron 12. The secondary winding 182 of the booster transformer 18 has a voltage doubler half-wave rectifier circuit 19 for rectifying the output thereof. The voltage doubler half-wave rectifier circuit 19 is constituted by a high-voltage capacitor 191 and two high-voltage diodes 192 and 193.
Here, such troubles may often happen that the microwave Oven is placed being in touch with the wall causing the ventilation port to be closed, or a foreign matter such as a chopstick or the like is bit by a cooling fan of the microwave oven, causing the cooling fan to be locked.
In order to prevent the IGBT for switching the inverter power source from being thermally broken down under the above situations, it has heretofore been attempted to halt the semiconductor IGBT by using a thermistor before it is thermally broken down or to decrease the power to prevent a rise in the temperature.
In this case, the temperature is detected by attaching the thermistor in a manner of:    (1) Fastening the thermistor together with a package by using a thermistor lead plug U with a spectacle terminal, which, however, can only be done by the manual work by a man resulting in an increase in the number of the steps and an increase in the cost;    (2) Inserting a radial thermistor in the leg of the IGBT, the radial thermistor being attached to the leg of the IGBT in only a subsequent step requiring manual labor, resulting in an increase in the number of steps, and being directly affected by the cooling air deteriorating the thermal time constant of the thermistor; or    (3) Fastening the thermistor to the heat-radiating fins by using separate screws to detect the temperature from the heat-radiating fins similarly resulting in an increase in the number of steps due to tightening the screws and an increase in the cost. Besides, the temperature is not directly detected from the IGBT but is detected through the heat-radiating fins, and both the detection precision and the sensitivity are not favorable.
Japanese Patent No. 2892454 (Patent Document 2) discloses an example of (2). FIG. 13B is a view illustrating a mounting method disclosed in the Patent Document 2. In FIG. 13B, reference numeral 306 denotes a printed board, 307 denotes heat-radiating fins, 308 denotes an IGBT, and 309′ denotes a thermistor. In this method, the radial thermistor is attached near to the printed board in a subsequent step requiring manual labor, resulting in an increase in the number of steps, and being directly affected by the cooling air, deteriorating the thermal time constant of the thermistor.
JP-A-2-312182 (Patent Document 1) also discloses an example of (3). FIG. 13A is a view illustrating a mounting method disclosed in the patent document 1 and illustrates a state where a thermistor is fastened to the heat-radiating fins by screws. In FIG. 13A, reference numeral 306 denotes a printed board, 307 denotes heat-radiating fins, 308 denotes an IGBT, and 309′ denotes a thermistor.
A heat-radiating portion of the IGBT 308 that generates a high temperature is secured to the heat-radiating fins 307, and its three legs are inserted in the through holes in the printed board and are soldered on the opposite side. Similarly, the thermistor 309′ is fastened by a screw to the heat-radiating fins 307 to take out the temperature data of the heat-radiating fins.
However, the method of fastening to the heat-radiating fins by screws also results in an increase in the number of steps and in an increase in the cost. Besides, the temperature is not detected directly from the IGBT but is detected from the heat-radiating fins, and both the detection precision and the sensitivity are not favorable.
Therefore, the present applicant has given attention to that the heat-radiating portion of the IGBT that generates high temperatures is secured to the heat-radiating fins and that the three legs thereof are inserted in the through holes of the printed board and are soldered on the opposite side (back side or the soldered side), and have discovered that when a chip thermistor is soldered to the leg portion of the IGBT or near to the leg portion thereof on the soldering side, particularly, on the emitter side, the thermistor which is a chip is quickly mounted by an automatic machine. The applicant has further discovered that the thermistor has a high thermal conductivity for the junction temperature of the IGBT, a small time constant, and directly receives a current flowing through the leg of the IGBT making it possible to detect the temperature that is dependent on the junction temperature of the IGBT with a short time constant (i.e., maintaining a high following performance), and that the thermistor is mounted not on the side of the cooling fins but on the soldering surface on the back side of the printed board without almost affected by the cooling air, which is convenient. Further, what makes a feature is that a chip thermister having a small heat capacity is attached to the leg portion of the IGBT having a small heat capacity or to a portion near the leg portion thereof featuring a small thermal time constant and enabling the power-down control to be accomplished at a high speed.
The conventional control circuit for controlling the IGBT, on the other hand, is employing the above thermistor arranged in a customary manner, having a large thermal time constant, and is not capable of conducting a quick control operation. Besides, the control circuit itself is not such that the temperature data of the thermistor are input to the inverter control circuit as will be described later, but are input to a central microcomputer to control the temperature.
FIGS. 10A and 10B are diagrams illustrating a circuit for controlling the start of a magnetron, wherein FIG. 10A is a circuit diagram and FIG. 10B is a diagram illustrating the operation of a comparator.
In FIG. 10A, a terminal (A) which is one of the two input terminals of a comparator CO1 receives a potential at a point P3 at where the collector voltage of the IGBT is divided by the voltage-dividing resistors R3 and R4, and the other terminal (B) receives 3 V since the switch S1 at the start is on the side of the terminal a. After the magnetron is heated and stabilized to assume a steady state, the change-over switch S1 is changed over to the side of the terminal b, and the terminal (B) receives a potential at a point Pc at where the voltage Vcc is divided by the voltage-dividing resistors R1 and R2.
Therefore, the circuit is turned off when the potential at the point P3 is smaller than 3 V at the start, and is turned on when the potential is higher than 3 V to repeat the turn on/off operation. Based on this data, the inverter control circuit 161 controls the ON/OFF duty of IGBT such that the potential at P3 becomes nearly in agreement is with 3 V, and the collector voltage of the IGBT becomes lower than that of during the steady-state operation.
During the steady-state operation, however, the terminal (B) of the comparator CO1 receives a potential Pc which is very higher than 3 V of at the start. Therefore, the inverter control circuit 161 works to increase the ON duty of ON/OFF control of IGBT so that the potential P3 becomes nearly in agreement with the potential Pc, and the collector voltage, too, of IGBT is elevated.
Here, however, though not illustrated, an increase in the ON duty is limited by the power control function which is possessed by the inverter control circuit 161 and works based on other input signals (e.g., input current data illustrated in the related art). As illustrated in FIGS. 10A and 10B, therefore, the potential Pc is maintained to be higher than the potential P3 at all times, and the output of the comparator CO1 is maintained to be turned on at all times.
As described above, the circuit for controlling the start of the magnetron of FIGS. 10A and 10B prevents an excess of voltage from being applied to the magnetron by controlling the collector voltage of IGBT to a predetermined value during the period (i.e., at the start) after the start of operation of the inverter circuit until the magnetron starts oscillating while flowing a heating current to the filament of the magnetron.
As will be described later, the power-down control of the invention utilizes the circuit for controlling the start of the magnetron of FIGS. 10A and 10B.
In case the fan suddenly stops revolving due to foreign matter that has bit the fan due to some cause, it was so far judged that the device has become faulty and the cooking had to be interrupted causing great psychological uneasiness to a person who is cooking to feel that the device has become faulty.
In the drawings, a reference numeral 7 refers to heat-radiating fins; 8 to an IGBT; 9 to a thermistor; 11 to a commercial power source; 12 to a magnetron; 13 to a rectifier circuit; 14 to a choke coil; 15 to a smoothing capacitor; 16 to an inverter; 161 to an inverter control circuit; 18 to a booster transformer; 181 to a first winding; 182 to a second winding; 183 to a winding for heating filament; 19 to a voltage doubler half-wave rectifier circuit; 307 to heat-radiating fins; 308 to a IGBT; and 309 to a thermistor.